Is there any warping during the print sufficient to disturb Z positioning significantly? I.e., is your hot end gouging into the part ever? While it looks like that probably isn't the case, just thought I'd mention it...

FWIW..., even though a part can be adhered to the table really well, it can still warp, but remain sufficiently attached to the table such that the HE will just keep ploughing through the part w/o the part coming unstuck. Kinda shocking when you first catch it happening...